(a) Field of the Invention
The present invention relates to an organic light emitting diode (“OLED”) display and a method of manufacturing the same.
(b) Description of the Related Art
Recent trends toward lightweight and thin personal computers and televisions require lightweight and thin display devices. Flat panel displays satisfying such a requirement are being substituted for conventional cathode ray tubes (“CRTs”).
Flat panel displays include liquid crystal displays (“LCDs”), field emission displays (“FEDs”), organic light emitting diode (“OLED”) displays, plasma display panels (“PDPs”) and various other types of displays.
Among the flat panel displays, the OLED display is the most promising because of its low power consumption, fast response time, wide viewing angle and high contrast ratio.
An OLED display is a self-emissive display device, which includes two electrodes and an organic light emitting layer interposed therebetween. One of the two electrodes injects holes and the other electrode injects electrons into the light emitting layer. The injected electrons and holes are combined to form excitons, which emit light as a product of their deactivation process.
OLED displays may be divided into passive matrix OLED displays and active matrix OLED displays as determined by the driving method used therein.
The passive matrix type OLED display includes a plurality of anode lines, a plurality of cathode lines intersecting the anode lines and a plurality of pixels, each pixel including a light emission layer. In order to operate the passive matrix type OLED one of the anode lines and one of the cathode lines is selected, which causes light emission in the pixel located at the intersection of the selected signal lines.
The active matrix type OLED display includes a plurality of pixels, each pixel including a switching transistor, a driving transistor and a storage capacitor, as well as an anode, a cathode and a light emission layer. In order to operate the active matrix type OLED, the driving transistor receives a data voltage from the switching transistor and drives a current, having a magnitude depending on the data voltage, into the light emission layer to cause light emission having intensity depending on the current.
In order to maximize the performance of the OLED display, characteristics of the switching thin film transistor and that of the driving thin film transistor, such as turn on voltage, etc., may be different from each other.
In particular, the switching thin film transistor may have good on/off characteristics, and the driving thin film transistor may have high mobility and stability for supplying sufficient currents for driving the OLED.
If the off current of the switching thin film transistor increases, then the data voltage transported to the driving thin film transistor may be reduced and cross-talk between pixels may be generated. If the driving thin film transistor has low mobility and low stability, display characteristics may deteriorate. The deteriorating display characteristics may result in unwanted phenomenon such as a reduction in the current transmitted to the light-emitting device, an image sticking phenomenon, operating life time reduction, etc.